The present invention relates to techniques for driving devices in half-bridge configurations. More specifically, the present invention provides techniques and circuitry for driving a high side device in a half-bridge configuration into an inductive load in a manner which reduces the effects of parasitic capacitances.
FIG. 1 is a schematic of a driver circuit 100 for the high side device in a half bridge circuit as described in U.S. Pat. No. 4,994,955 for HALF-BRIDGE DRIVER WHICH IS INSENSITIVE TO COMMON MODE CURRENTS issued on Feb. 19, 1991, the entirety of which is incorporated herein by reference for all purposes. The lower half of the schematic is the transmitter portion 76 of a level shifter which converts pulsatile inputs at VON and VOFF into current pulses ION and IOFF, respectively. The upper half of the schematic is the receiver portion 78 of a high side driver which generates the gate to source voltage for the high side device (not shown) between lines 38 and 40. Current pulse ION is transmitted alone to place the high side device in a conductive state, while current pulse IOFF is transmitted alone to place the high side device in a non-conductive state.
To provide insensitivity to common mode currents IC (due to parasitic capacitances 82), a double differential amplifier comprising transistors T1 and T2 and matched resistors R5 and R6 is employed. The double differential amplifier produces substantially identical voltages at V1 and V2 in response to the common mode currents which will not turn on transistors T1 and T2. By contrast, the current pulse ION flowing only in line 70 turns on T1 while the current pulse IOFF flowing only in line 72 turns on T2. The resulting set and reset voltages (Vs and VR) are input to an RS flip-flop 96 which provides the gate drive to the high side device via buffer 106.
Unfortunately, the solution provided by the circuit of FIG. 1 does not address other undesirable effects of parasitic capacitance 82. For example, if the high side device being driven by the circuit of FIG. 1 drives an inductive load or filter, the resonant current in the inductor can drive the device output toward its positive rail. This positive going output voltage swing, in turn, causes current to flow in parasitic capacitances 82 (these capacitances being charged via either R1, R2, T9, and T12). As this current flows, the VD supply rail moves up in voltage. That is, VD moves up as the output of the high side device approaches its rail because the high side device output is coupled to the VD rail via a xe2x80x9cbootstrapxe2x80x9d capacitance.
As voltages are developed across R1 and R2 due to the parasitic currents, the xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d signals into the high side driver are simultaneously activated. The magnitude of these currents can be such as to active the clamping mechanism implemented with transistors T11 and T12. During the time when T11 and T12 are activated, input pulses are ignored. As a result, legitimate xe2x80x9conxe2x80x9d pulses could be ignored because of the parasitic currents. This is clearly an undesirable result.
A potential solution to this problem could be to reduce the values of resistors R1 and R2 in an attempt to reduce the likelihood of this clamping effect. However, this would require more current to operate the circuit and does not guarantee that the problem is eliminated for all values of output inductors, output transistors, and control signal pulse widths. It should also be noted that eliminating the pulse generators driving VON and VOFF alone does not solve the problem as the xe2x80x9conxe2x80x9d signal may still be shorter in duration than the duration of the clamping.
It is therefore desirable to provide techniques by which it can be ensured that an xe2x80x9conxe2x80x9d signal for the high side device in a half-bridge configuration is never unintentionally ignored.
According to the present invention, techniques and circuitry are provided which compensate for the effects of parasitic currents in half-bridge driver circuits such that xe2x80x9conxe2x80x9d signals are not unintentionally ignored. In addition, and according to specific embodiments, the techniques of the present invention prevent the occurrence of resonance oscillations in the MUTE mode of audio amplifiers which may result, at least in part, from parasitic capacitances. According to a specific embodiments, these goals are achieved by generating a compensating current which effectively cancels the effects of the parasitic current generated in the parasitic capacitance of the driver circuit""s input device. This compensating current is generated using a compensating device configured similarly to the driver circuit""s input device. The compensating current is generated in the compensating device""s parasitic capacitance due to the same condition which causes the parasitic current in the input device. A current mirror is then used to provide the same magnitude current to the parasitic capacitance of the driver circuit""s input device, thereby canceling at least part of the effect of the parasitic current.
Thus, the present invention provides a circuit for compensating for a first parasitic current corresponding to a first parasitic capacitance associated with a first switch. A second switch is configured substantially the same as the first switch, the second switch having a second parasitic capacitance associated therewith. A current mirror coupled to the second switch generates a compensating current in response to a second parasitic current corresponding to the second parasitic capacitance. The compensating current compensates for at least a portion of the first parasitic current.
A further understanding of the nature and advantages of the present invention may be realized by reference to the remaining portions of the specification and the drawings.